If you’re not the games developer or publisher then you do not have the ability to update box art, as that can only be done by them.
If you think a games box art should be updated then contact the developers and ask them to go through the process of claiming ownership and update the art as explained here: https://dev.twitch.tv/docs/companies
If the games were legacy titles from now defunct studios then it would make sense for 3rd parties to be able to update the box art, but both of those games you mentioned are new releases, from game studios that are still very much in business.
As I previously mentioned your best way forward would be to reach out to the developers and explain the issue with the lack of box art on Twitch and how they resolve it, as it’s for them to decide what artwork to use (and for all you know, perhaps their marketing team doesn’t want to use certain artwork there which may be why it’s left blank).